In fabrication of fluorescent lamps, a phosphor layer is applied to the interior surface of a glass envelope using a water base or organic base paint-like suspension of phosphor powder. Environmental considerations favor the use of an aqueous (water base) suspension. The phosphor layer may be applied directly to the glass or may alternatively be applied to a previously applied coating of, for example, phosphor or reflective material.
Although the composition of the phosphor coating suspension may vary from lamp manufacturer to lamp manufacturer, the suspension usually includes, in addition to the phosphor, a film forming binder, solvent(s) for the binder, and, if necessary, surfactants, defoamers, and wetting agents. The coating suspension may further include submicron particle size alumina, e.g., Aluminum Oxide C (ALON C), manufactured by Degussa, Inc.
In triphosphor coating suspensions, an ALON C content of 2 to 10% weight percent of the suspension is typical.
Many of the non-phosphor components of the coating suspension interfere with efficient lamp operation and longevity, and are therefore removed from the phosphor coating during the manufacturing process by high temperature air oxidation in a manufacturing step known in the art as "lehring."
The submicron particle size alumina is not removed during the lehring step. The submicron particle size alumina assists in the formation of a uniform adherent phosphor layer on the lamp surface.
Butler, in his book, Fluorescent Lamp Phosphors, Technology and Theory, Penn State University Press (1980), describes lamp coating technology and its evolution from the nitrocellulose and ethylcellulose-type lacquers to the newer polymeric binders used with water base suspension systems.
Depending upon the chemical composition of the phosphor and method of preparation, phosphors may exhibit some differences in performance depending upon whether a water base or organic base suspension is used to apply the phosphor coating to the bulb. Usually these differences are not significant unless the phosphor has a tendency to react chemically with one of the suspension components.